1. Field of the Invention
The present invention relates to a magnetic core having performance and stability properties which make the core particularly suitable for utility in power generation parts such as stators, rotors, armatures and actuators or any device whose function is dependent upon an efficient magnetic core, i.e., a magnetic core having minimal magnetic hysteresis and no or little eddy current formation.
2. Discussion of the Background
Magnetic materials generally fall into two classes which are designated as magnetically hard substances which may be permanently magnetized or soft magnetic materials which may be reversed in magnetism at low applied fields. It is important in soft magnetic materials that energy loss, normally referenced as “core loss” is kept to a minimum whereas in hard magnetic materials it is preferred to resist changes in magnetization. High core losses are therefore characteristic of permanent magnetic materials and are undesirable in soft magnetic materials.
Soft magnetic core components are frequently used in electrical/magnetic conversion devices such as motors, generators and transformers and alternators, particularly those found in automobile engines. The most important characteristics of soft magnetic core components are their maximum induction, magnetic permeability, and core loss characteristics. When a magnetic material is exposed to a rapidly varying magnetic field, a resultant energy loss in the core material occurs. These core losses are commonly divided into two principle contributing phenomena: hysteresis and eddy current losses. Hysteresis loss results from the expenditure of energy to overcome the retained magnetic forces within the core component. Eddy current losses are brought about by the production of induced currents in the core component due to the changing flux caused by alternating current (AC) conditions.
The use of powdered magnetic materials allows the manufacture of magnetic parts having a wide variety of shapes and sizes. Conventionally, however, these materials made from consolidated powdered magnetic materials have been limited to being used in applications involving direct currents. Direct current applications, unlike alternating current applications, do not require that the magnetic particles be insulated from one another in order to reduce eddy currents.
Conventionally, magnetic device parts are constructed from powders by compaction of the powders to a defined shape and then sintering the compact at temperatures of 600° C. or higher. Sintering the part following compaction, is necessary to achieve satisfactory mechanical properties in the part by providing particle to particle bonding and hence strength. However, sintering may cause volume changes and results in a manufacturing process with poor dimensional control.
In other conventional processes designed to prepare parts having minimum eddy current losses, the magnetic particles are coated with thermoplastic materials before pressing. The plastic is provided to act as a barrier between the particles to reduce induced eddy current losses. However, in addition to the relatively high cost of such coatings, the plastic has poor mechanical strength and as a result, parts made using plastic-coated particles have relatively low mechanical strength. Additionally, many of these plastic-coated powders require a high level of binder when pressed. This results in decreased density of the pressed core part and, consequently, a decrease in magnetic permeability and lower induction. Additionally, and significantly, such plastic coatings typically degrade at temperatures of 150-200° C. Accordingly, magnetic parts made in such manner are generally limited to utility in low stress applications for which dimensional control is not critical.
Thus, there remains a need for magnetic powders to produce soft magnetic parts, having increased green strength, high temperature tolerance, and good mechanical properties, which parts have minimal or essentially no core loss.
Conventionally, ferromagnetic powders have been employed for the production of soft magnetic core devices. Such powders are generally in a size range measured in microns and are obtained by a mechanical milling diminution of a bulk material. Superparamagnetic nanoparticle materials having particle size of less than 100 nm have found utility for magnetic record imaging, as probes for medical imaging and have been applied for targeted delivery of therapeutic agents. However, the utilization of superparamagnetic powders for production of core magnetic parts has until now, been limited.
For example, Toyoda et al. (U.S. 2011/0104476) describe a soft magnetic material of iron or an iron alloy particle having a grain size of from 5 to 400 μm which is provided with an oxide insulative coating including silicon oxide. The coated particles are mixed with an organic substance which is a non-thermoplastic resin and at least one of a thermoplastic resin and a higher fatty acid. The content of the organic substance in the mixed material is from 0.001 to 0.2% by mass. The mixed material is compression molded and then subjected to a heat treatment at a temperature between the glass transition temperature and the thermal decomposition temperature of the non-thermoplastic resin. The molded and heat treated structure is indicated to be useful for electric and electronic components such as a motor core or a transformer core.
Liu (U.S. 2010/0054981) describes a system of magnetic nanoparticles which is a composite of a hard magnetic material and a soft magnetic material. For example, a “bimagnetic” FePt/Fe3O4 nanoparticle is described. Liu describes “warm compaction of the material to produce a bulk nanocomposite magnet.
Hattori et al. (U.S. 2006/0283290) describe silica coated, nitrided iron particles having an average particle diameter of 5 to 25 nm. The particles are “substantially spherical” and are useful for magnetic layers such as a magnetic recording medium.
Ueta et al. (U.S. 2003/0077448) describes a ferromagnetic raw metal powder (primarily iron) having a coating of various oxide materials including silicon. Claim 1 provides a ferromagnetic powder which is surface coated with a silicone resin and a pigment. The coated particle has a diameter on the order of 100 microns. Warm pressing of the powder to produce a core is described as well as annealing of a core at elevated temperature.
Tokuoka et al. (U.S. Pat. No. 7,678,174) describe an iron based powder particle having an iron or iron alloy core and an oxide type insulating coating, including silicon oxide. An ester wax is also added to the particle surface. The coated powder particles are on the order of 200 microns in size as described in Example 1. The lubricated powder is pressure molded to form a molded body and the molded body heat treated.
Blagev (U.S. Pat. No. 5,512,317) describes an acicular magnetic iron oxide particle having a magnetic iron oxide core and a shell containing a silicate compound and cobalt (II) or iron (II) compound as a dopant. The doped acicular particles have a length typically of about 0.15 to 0.50 μm and are employed in magnetic recording media.
Nomura et. al. (U.S. Pat. No. 5,451,245) describes acicular magnetic particles having a largest dimension of about 0.3 μm which are suitable for magnetic recording media. Hydrated iron oxide particles are first coated with an aluminum or zirconium compound, then heated to form a hematite particle. This formed particle is then coated a second time with an aluminum compound followed by a reduction treatment. Silicon compounds may be included in either coating to enhance the properties of the particle.
Tamai et al. (U.S. Pat. No. 5,225,281) describes a coated iron particle having an iron core and a coating containing zirconium, aluminum and silicon. The particle has an average major axis length of 0.1 to 0.5 μm and an average minor axis length of 0.01 to 0.035 μm. Utility of the particle as a magnetic recording medium is described.
Kadono et al. (U.S. Pat. No. 4,920,010) describes an acicular ferromagnetic metal powder particle which is an iron or mainly iron particle having first a silicon compound coating, then a coating of a nonferrous transition metal compound. The acicular particle has a major axis diameter of 0.25 microns. Utility of the particle as a magnetic recording medium is described.
Soileau et al. (U.S. Pat. No. 4,601,765) describes a core obtained by compaction of iron powder which has been coated with an alkali metal silicate and then a silicone resin polymer. The iron particles to which the coating is applied have a mean particle size of 0.002 to 0.006 inches. The core is prepared by compaction of the powder at greater than 25 tons per square inch and then annealing the pressed component.
Tokuoka (U.S. Pat. No. 4,309,459) describes a method to prepare an acicular iron or iron oxide powder where an aqueous iron oxide is mixed with a water soluble silicate and hydrothermally reacted at high pressure. The resulting acicular powder is indicated to be useful as a magnetic recording medium.
Yu et al. (J. Phys. Chem. C 2009. 113, 537-543) describes the preparation of magnetic iron oxide nanoparticles encapsulated in a silica shell. Utility of the particles as magnetic binding agents for proteins is studied.
Tajima et al. (IEEE Transactions on Magnetics, Vol. 41, No. 10, October, 2005) describes a method to produce a powder magnetic core described as warm compaction using die wall lubrication (WC-DWL). According to the method an iron powder coated with a phosphate insulator was compacted under a pressure of 1176 MPa at a temperature of 423° K. to produce a core type structure.
Sun et al. (J. Am. Chem. Soc., 2002, 124, 8204-8205) describes a method to produce monodisperse magnetite nanoparticles which can be employed as seeds to grow larger nanoparticles of up to 20 nm in size.
Bumb et al. (Nanotechnology, 19, 2008, 335601) describes synthesis of superparamagnetic iron oxide nanoparticles of 10-40 nm encapsulated in a silica coating layer of approximately 2 nm. Utility in power transformers is referenced, but no description of preparation of core structures is provided.
None of the above references disclose or suggest a monolithic magnetic core constructed by heated compression of nanoparticular iron oxide encapsulated in a silicon dioxide coating shell, wherein the particles are directly compacted without addition of lubricant or other material to facilitate particle adherence.